Print timer now stops when it sees the last extruder temperature being shutdown

Marlin/Marlin_main.cpp

         setTargetHotend1(temp == 0.0 ? 0.0 : temp + duplicate_extruder_temp_offset);
     #endif
   }
+
+  // Detect if a print job has finished.
+  // When the target temperature for all extruders is zero then we must have
+  // finished printing.
+  if( print_job_start_ms != 0 ) {
+    bool all_extruders_cooling = true;
+    for (int i = 0; i < EXTRUDERS; i++) if( degTargetHotend(i) > 0 ) {
+      all_extruders_cooling = false;
+      break;
+    }
+
+    if( all_extruders_cooling ) print_job_stop_ms = millis();
+  }
 }
 
 #if HAS_TEMP_0 || HAS_TEMP_BED || ENABLED(HEATER_0_USES_MAX6675)
  *       Rxxx Wait for extruder(s) to reach temperature. Waits when heating and cooling.
  */
 inline void gcode_M109() {
+  float temp;
   bool no_wait_for_cooling = true;
 
   if (setTargetedHotend(109)) return;
   if (marlin_debug_flags & DEBUG_DRYRUN) return;
 
-  LCD_MESSAGEPGM(MSG_HEATING);
-
   no_wait_for_cooling = code_seen('S');
   if (no_wait_for_cooling || code_seen('R')) {
-    float temp = code_value();
+    temp = code_value();
     setTargetHotend(temp, target_extruder);
     #if ENABLED(DUAL_X_CARRIAGE)
       if (dual_x_carriage_mode == DXC_DUPLICATION_MODE && target_extruder == 0)
     #endif
   }
 
+  // Only makes sense to show the heating message if we're in fact heating.
+  if( temp > 0 ) LCD_MESSAGEPGM(MSG_HEATING);
+
   #if ENABLED(AUTOTEMP)
     autotemp_enabled = code_seen('F');
     if (autotemp_enabled) autotemp_factor = code_value();
     if (code_seen('B')) autotemp_max = code_value();
   #endif
 
+  // Detect if a print job has finished.
+  // When the target temperature for all extruders is zero then we must have
+  // finished printing.
+  if( print_job_start_ms != 0 ) {
+    bool all_extruders_cooling = true;
+    for (int i = 0; i < EXTRUDERS; i++) if( degTargetHotend(i) > 0 ) {
+      all_extruders_cooling = false;
+      break;
+    }
+
+    if( all_extruders_cooling ) {
+      print_job_stop_ms = millis();
+      LCD_MESSAGEPGM(WELCOME_MSG);
+    }
+  }
+
   // Exit if the temperature is above target and not waiting for cooling
   if (no_wait_for_cooling && !isHeatingHotend(target_extruder)) return;
 

Marlin/dogm_lcd_implementation.h

 
     u8g.setPrintPos(80,48);
     if (print_job_start_ms != 0) {
-      uint16_t time = (millis() - print_job_start_ms) / 60000;
+      uint16_t time = ((print_job_stop_ms > print_job_start_ms)
+                       ? print_job_stop_ms : millis()) / 60000 - print_job_start_ms / 60000;
       lcd_print(itostr2(time/60));
       lcd_print(':');
       lcd_print(itostr2(time%60));

Marlin/temperature.cpp

   for (int i = 0; i < EXTRUDERS; i++) setTargetHotend(0, i);
   setTargetBed(0);
 
+  // If all heaters go down then for sure our print job has stopped
+  if( print_job_start_ms != 0 ) print_job_stop_ms = millis();
+
   #define DISABLE_HEATER(NR) { \
     target_temperature[NR] = 0; \
     soft_pwm[NR] = 0; \

Marlin/ultralcd_implementation_hitachi_HD44780.h

   #define LCD_I2C_PIN_D5  5
   #define LCD_I2C_PIN_D6  6
   #define LCD_I2C_PIN_D7  7
-  
+
   #include <Wire.h>
   #include <LCD.h>
   #include <LiquidCrystal_I2C.h>
         else {
           if (!axis_homed[X_AXIS])
             lcd_printPGM(PSTR("?"));
-          else 
+          else
             #if DISABLED(DISABLE_REDUCED_ACCURACY_WARNING)
               if (!axis_known_position[X_AXIS])
                 lcd_printPGM(PSTR(" "));
         else {
           if (!axis_homed[Y_AXIS])
             lcd_printPGM(PSTR("?"));
-          else 
+          else
             #if DISABLED(DISABLE_REDUCED_ACCURACY_WARNING)
               if (!axis_known_position[Y_AXIS])
                 lcd_printPGM(PSTR(" "));
     else {
       if (!axis_homed[Z_AXIS])
         lcd_printPGM(PSTR("?"));
-      else 
+      else
         #if DISABLED(DISABLE_REDUCED_ACCURACY_WARNING)
           if (!axis_known_position[Z_AXIS])
             lcd_printPGM(PSTR(" "));
     lcd.setCursor(LCD_WIDTH - 6, 2);
     lcd.print(LCD_STR_CLOCK[0]);
     if (print_job_start_ms != 0) {
-      uint16_t time = millis() / 60000 - print_job_start_ms / 60000;
+      uint16_t time = ((print_job_stop_ms > print_job_start_ms)
+                       ? print_job_stop_ms : millis()) / 60000 - print_job_start_ms / 60000;
       lcd.print(itostr2(time / 60));
       lcd.print(':');
       lcd.print(itostr2(time % 60));